矽廢料應用在鋰離子電池負極材料之製備與特性分析

Preparation and Characterization of Si/C Composite Anode Derived from Si Waste for Lithium Ion Batteries

謝政哲(Cheng-Che Hsieh)

指導教授 ： 劉偉仁

中原大學/工學院/化學工程研究所/博士(2019年)

https://doi.org/10.6840/cycu201900909

摘要

鋰離子電池發展至今，在負極材料中以碳材料在其中扮演著重要的角色，也是使用最多的負極材料，然而，為了進一步提高石墨負極材料的比電容量與各項電化學性能的表現，許多研究也開始針對高能量密度的矽負極電池材料進行發展，因矽材料具有環境的破壞性低，在地球上儲存量高達23%與其較佳的穩定性，所以，為了能因應未來的儲能設備與動力電池的開發，矽基負極材料的研究與產業化變得不可或缺，在下一世代的動力中成為亮點。
本篇論文針對矽負極材料的兩大缺陷進行改良，針對不良的導電特性與劇烈體積膨脹進行表面改質與結構設計，進一步提升矽負極材料充放電循環壽命的影響。研究中將分為三個階段進行探討:
第一部分我們以氮摻雜的方式對Si/C複合材料進行摻雜，研究中我們發現直接對複合材料進行摻雜後，在XPS分析中可以發現Si-N鍵結的特徵峰出現，整體摻雜量最高可以達到3.67 mol.%。在循環壽命測試下，氮摻雜Si/C複合材料經過0.5C的條件下測試100圈後，還擁有1238 mAh g−1的電容量，而在快充測試下，我們發現在1C的電流速度下來擁有877 mAh g−1的電容量，其電化學結果皆優於未摻雜前的矽負極，同時我們也透過四點探針進行量測摻雜矽負極，由結果中也顯示出片電阻與導電率皆大幅度提升，根據以上結果，對於矽本身導電性不佳的缺點，有效的以氮摻雜的改質方法得到改善。
第二部分我們則是以鹼處理的方式對矽進行蝕刻，製備出多孔結構與矽結構的非晶化，讓鋰離子在遷入與遷出時能更加容易，進一步降低矽的劇烈體積膨脹與收縮。我們透過拉曼的探討中，經過鹼處理後矽的特徵峰明顯發生了改變，由晶相矽生成部分非晶相矽，在FTIR測試下，我們也可以觀察到表面較強的Si-O鍵結轉變成較弱的Si-O鍵結。在BET測試中也可明顯發現表面積大幅度的提升，由原本的8.6上升至234 m2 g-1。透過電化學的方式進一步討論，不僅僅循環壽命大幅度提升，也在微分電容的圖中觀察到鋰遷入矽負極的變化，同時透過SEM觀察下，極板整體的膨脹程度大幅度降低，極板碎裂的程度也明顯減少。因此對於矽本身的劇烈體積膨脹收縮現象，可以有效透過鹼處理的方式達到緩解的功用，進一步提升矽負極材料的應用可能性。
第三部分，我們嘗試將矽負極材料與不同碳來源進行複合並探討其電性表現，其中我們使用的碳來源為片狀的KS6，一維結構的奈米碳管，球狀的碳黑Super-P與透過低溫破碎後的寡層石墨烯微片。透過單純添加片狀石墨後，因石墨具有緩衝效果，在電性表現上即可到有效的改善，但快充能力卻不佳，因此我們透過探討添加不同量的奈米碳管後，對矽負極材料整體的快充能力大幅度提升，同時奈米碳管也可提供抑制體積膨脹效果，而在Super-P添加下可以填補矽顆粒間的縫隙，同時可當作矽顆粒間的導電橋梁，在快充測試下大幅度提升矽的快充能力。另外一方面，當我們將石墨換成寡層石墨烯後，不僅僅可以發現在第一圈鋰更容易與矽進行反應，也有效利用了石墨烯的柔軟特性抑制矽顆粒膨脹收縮，因此在循環壽命測試下，可擁有300圈的穩定性與較佳的電容量表現。

關鍵字

矽 ； 化學處理法 ； Si/C複合材料 ； 鋰離子電池 ； 負極

並列摘要

Lithium ion batteries are one of the most important devices for the electrochemical energy storage system because of their high energy density and good cycle life. The applications and the corresponding products of graphite as anode for lithium ion batteries are no longer limited to the 3C products. Therefore, the higher capacity density and higher working voltage are being developed.
Among reported anode materials, silicon has been regarded as a promising anode material because of its large theoretical capacity of 4200 mAh/g. Unfortunately, the major problems of Si anode involve large volume expansion and poor electrical conductivity for lithium insertion/extraction. Here, we will report research about Si anode and the corresponding Si modification of Si anode for Li ion battery applications.
From the view point of Si poor electric conductivity, we propose an one-step method for synthesizing nitrogen-doped silicon/carbon composite as an anode material for lithium-ion batteries. The electrochemical results indicate that the carbon and doped nitrogen content in Si considerably influence its electronic conductivity and structural stability. The X-ray photoelectron spectroscopy results indicate that approximately 0.73–3.67 mol.% of nitrogen atoms were successfully doped in Si/C composite. Nitrogen-doped Si/C composite exhibited a high reversible capacity of 1238 mAhg−1 after 100 cycles at 0.5 C. Moreover, nitrogen-doped Si/C composite exhibited a higher rate capability than Si/C. The N-doped sample had a charge capacity of approximately 877 mAhg−1 at a current rate of 1 C. Furthermore, by using four-point probe tests, the electronic conductivity of nitrogen-doped Si/C composite and bare electrodes were obtained as 19100 and 14298 Scm−1, respectively. The obtained results indicate that the electrochemical performance of a silicon electrode can be considerably improved through the combination of N doping and pitch coating.
In the second part of this thesis, we propose a NaOH-etched and a carbon coating processes for Si anode. The two-step approach not only enhance electronic conductivity of Si/C by carbon coating to enhance columbic efficiency of Si during cycling processes but also architecturally create porous structure of Si to buffer volume expansion. The results indicate that both solid content of Si and etching time play vital roles on Si anode in terms of structural stability and cycling performance. Raman spectra demonstrate a transition from crystalline Si to amorphous Si during etching process. FTIR results indicate that the Si-O bonding types of Si powder were changed from symmetry to asymmetry. The BET analysis indicates that the specific surface of modified Si increased from 8.6 to 234 m2/g. After condition-optimization, modified Si/C composite delivered a reversible capacity of 780 mAh/g for more than 200 cycles at 0.5C. The obtained results highly indicate that through the combination of modified process and pitch coating, electrochemical performance of Si electrode are greatly improved.
Si/carbon-based composite is an ideal strategy to overcome the critical issues of Si anodes Combining the advantages of high conductivity carbon-based materials and high capacity Si can take advantage of their merits fully. Inspired by that, in the third chapter, we developed Si/KS6/C, Si/KS6+CNTs/C and Si/KS6+Super P/C composite with high rate performance and cyclic stability. The carbon nanotubes (CNTs) can effectively enhance the connection between Si-Si particles while Super P can provide higher electric conductivity for the Si anode. Furthermore, we developed a structure of modified Si with FLG that had higher electric conductivity than graphite sheet. MSiG/C-15% (G:Mo-KS6) exhibited a high reversible capacity of 757 mAhg−1 after 300 cycles at 0.5 C. It is believed that these novel anode composite could potentially have opportunities to be promising candidates as anode materials for Li-ion batteries in the future.

並列關鍵字

Si ； chemical modification ； Si/C ； Lithium ion battery ； anode

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